The present invention relates to the use of connecting pins for securing machine parts together, particularly in situations where the connecting pins should be flush with opposed surfaces of the parts which are connected together.
In the assembly of many machines, it is convenient to employ connecting pins for securing machine parts together. Frequently, it is desirable that the ends of these connecting pins be flush with associated surfaces of the connected parts. Such connecting arrangements are employed, for example, to fastened control stage turbine blade units to a turbine rotor or disc.
Such an arrangement is shown in FIG. 1, where a turbine blade unit 2 is fastened to a disc or rotor 4 carried on a rotor shaft 6. Turbine blade unit 2 is provided with a root portion composed of a plurality of plates 8 which fit into circumferential grooves in disc 4. Disc 4 and plates 8 are provided with passages, for example circular holes, for receiving connecting pins 10. When the parts are connected together, pins 10 will extend between opposed, parallel surfaces 12 which here are radial surfaces of disc or rotor 4.
For the purpose illustrated in FIG. 1, and for similar purposes in other machines, connecting pins 10 must fit tightly enough to prevent relative motion between the parts which they connect together, even if one of the parts is subjected to intermittent shock forces. Moreover, it is desirable, and often essential, that the length of the pins correspond precisely to the distance between surfaces 12 and that the ends of the pins fit flush with those surfaces.
If the ends of the pins were not flush with the end surfaces of the assembly, this can create turbulence during high speed turbine operation, which turbulence would contribute to the aerodynamic resistance of the rotating parts and produce a certain degree of wear. Moreover, protruding pins may strike stationary surfaces within the turbine and would mar the appearance of the assembly.
This means that each pin must not only be of the correct length, but must be inserted to precisely the correct depth; if a pin of correct length is inserted too far, or if the pin is too long, it will project from one surface of the parts which are connected together, and this will frequently be unacceptable. Since the manufacture of connecting pins to initially have precisely the desired length for a given installation is impractical, current practice is to manufacture such pins so that they are initially longer than required. Each pin is then inserted so that both ends project from the parts being connected together. In the case of the device shown in FIG. 1, such pins would project beyond both surfaces 12. One manner of inserting such pins is to freeze the pins, for example in liquid nitrogen, after which the pins are quickly inserted and allowed to return to ambient temperature, while expanding, to produce the desired interference fit. Then, the excess portion of each pin is machined away at both surfaces 12.
This procedure has a number of inherent drawbacks. Firstly, machining is a slow and expensive process particularly since, in many situations, the connecting pins must be of a high strength material with a suitable coefficient of thermal expansion, and such materials are relatively hard. In addition, the machining process is one of the last operations performed in the assembly of a structure such as that shown in FIG. 1 and any delays in this process can have a significant impact on completion of the assembly on schedule. Finally, the connecting pin material will frequently be harder than that of the parts which are connected together so that when the ends of a connecting pin are machined flush with the surfaces of the connecting parts, an unacceptable amount of the connecting part surfaces may be machined away at the same time.
If it were attempted to obviate these difficulties by giving a pin the desired length before insertion, it is possible for a pin to be inserted too far with the result that one end of the pin would be recessed and the other end protruding. Such an installation is not acceptable. In installations of the type here under consideration, it is not possible to simply force the pin back in the opposite direction because the pin will have already reached a temperature at which it is fixed in place.
After a pin has been properly inserted, the tightness of its fit in its associated passage must be tested. Current practice involves placing an instrument against one end of the pin and then striking the instrument with a hammer, an effort being made to strike the pin with a selected force. Inherently, it is difficult to control the force produced by a hammer blow. Therefore, when this technique is employed, it will frequently occur that the force of the hammer blow is too low or too high. In the former case, a pin whose fit is not sufficiently tight will appear to be acceptable, while in the latter case, an acceptable pin may be dislodged. Frequently, if a pin is dislodged, it can not be repositioned because there is not sufficient room adjacent to the opposite end of the pin for introduction for an appropriate tool.